Calculating "Same Store" Sales in PowerPivot

I love the PowerPivot forum over at Mr. Excel for many reasons, one of which is that it gives me useful, real-world inspiration for blog posts. Recently, I saw someone asking about how to calculate "same store sales."

Simply put, same store sales means comparing sales of ONLY stores that are open today and were also open at the same time last year (or last month, last quarter, etc.).

In other words, it's a "year over year" (or month over month, etc.) comparison measure that only looks at stores that were open last year, and are still open today.